As is known, integrated power devices designed to withstand voltages of several hundreds of volts, such as for example insulated-gate bipolar transistors (IGBTs) or power-MOS transistors, are normally controlled by driver devices which, among other things, operate to prevent overvoltages or undesirable oscillations on the high-voltage terminals, which may cause serious problems.
For example, in the case of electronic-ignition equipment used for sparking combustion in internal-combustion engines, IGBTs are normally used as power switches for supplying the primary windings of the high-voltage transformers. In greater detail, upon switching-on of the power switches, the primary winding of one of the high-voltage transformers is supplied with a constant battery voltage and hence is traversed by a current which increases in time. A clamp circuit intervenes to limit said current when the energy stored in the primary winding is sufficient to cause a spark between the electrodes of a spark-plug connected to the secondary winding of the same transformer. The opening of the circuit by the power switch causes a voltage peak (normally comprised between 400 V and 600 V), which, amplified by the transformation ratio of the transformer, causes the spark. In the example described, the voltage on the collector of the IGBT (high-voltage terminal) must be controlled both upon switching-off of the IGBT itself, to prevent any possible breakdown, and when the limiting circuit intervenes; in the absence of control, in fact, so large oscillations may occur as to cause undesirable sparks.
Clearly, in order to be able to carry out the control, it is necessary to supply the driver device associated to the power device with a signal representing the voltage present on the high-voltage terminal (the collector terminal, in the case of IGBTs); on the basis of this signal, the driving device acts on a control terminal of the power device.
For this purpose, a known technique is to integrate a high-voltage sensor within the power device. In particular, two solutions have been proposed, which will be briefly described as follows.
A first solution is illustrated in FIG. 1, which shows an IGBT 1 made in a semiconductor body and comprising a collector region 3, a conduction region 5, body regions 6, emitter regions 7, a gate region 8, an equipotential region 9, and a protection circuit 10.
The collector region 3 is formed in a substrate of the body 2, here of P+ type, and has a face coated with a metal layer forming a collector contact 11. The conduction region 5, which is of N− type, extends above the collector region 3 and is separated therefrom by a junction layer 13, of N+ type. Furthermore, the collector region 3 and the conduction region 5 are high-voltage regions.
The body regions 6, of P+ type, are housed in the conduction region 5 and emerge on a surface 5a of the conduction region itself. In turn, the body regions 6 house the emitter regions 7, which are of an N+ type and which are also substantially flush with the surface 5a. Preferably, all of the emitter regions 7 are connected to a single emitter contact 14.
The gate region 8, which is made of polysilicon, is insulated from the conduction region 5 by means of a thin layer of gate oxide 15, and from the emitter contact 14 by means of an insulating layer 16. Furthermore, the gate region 8 comprises a first portion 8a, provided with a gate contact 19, and second biasing portions 8b (the first and the second portions 8a, 8b are connected together; however, the connection is not visible in the cross-sectional view of FIG. 1). The second portions 8b of the gate region 8 extend between adjacent body regions 6 and partially overlap the emitter regions 7.
The equipotential region 9, which is normally metallic, is in direct electrical connection with the conduction region 5 and extends on its surface 5a around the IGBT 1. The voltage of the conduction region 5 and of the equipotential region 9 differ from the voltage of the collector region 3 only by the forward voltage present on the PN junction formed by the collector region 3 and by the junction region 13. For this reason, the equipotential region 9 is used as a sensor for detection of the collector voltage.
The protection device 10 is connected to the gate contact 19 and to the equipotential region 9 and comprises a cascade of Zener diodes 20 connected in pairs in back-to-back configuration, i.e., having cathode terminals in common. In practice, the Zener diodes are formed by means of first and second strips 21, 22 of polysilicon, which are contiguous and have alternated P type and N type conductivity (see FIG. 2). More precisely, each first strip 21, having for example P type conductivity, is contiguous to two second strips 22 having N type conductivity; instead, each second strip 22 is contiguous to two first strips 21. The Zener diodes 20 are formed by the junctions 23 between adjacent strips 21, 22. Consequently, each first strip 21, having P type conductivity, constitutes a common anode terminal of two adjacent Zener diodes 20; likewise, each second strip 22, having N type conductivity, constitutes a common cathode terminal of two adjacent Zener diodes 20. The protection device 10 is moreover made on a thick field-oxide region 25 and hence is electrically connected to the conduction region 5 only through the equipotential region 9. In practice, the protection device 10 limits the voltage between the gate region 8 and the equipotential region 9 to a maximum value equal to the sum of the reverse breakdown voltages of the equi-oriented Zener diodes 20 (i.e., half of the total number of the Zener diodes 20).
The solution described has, however, certain limits, principally in that the terminal used as high-voltage sensor, i.e., the equipotential region 9, is in turn a high-voltage terminal and hence can be connected only to control devices capable of withstanding and processing voltages of several hundreds of volts.
A different solution, illustrated in FIG. 3, where parts that are the same as the ones already illustrated bear the same reference numbers, envisages the use, as a voltage sensor, of a junction field-effect transistor (JFET) 27, operating as a high-voltage nonlinear resistor. In greater detail, the JFET 27 has an emitter contact 28, formed on a portion 5a of the conduction region 5, which is delimited laterally by two body regions 6. In addition, the body regions 6 and the conduction region 5 are used as gate regions and, respectively, as drain region of the JFET 27, while the emitter contact 14 functions also as gate contact. In practice, the current flowing through the JFET 27 is modulated by the voltage present between the conduction region 5 and the emitter contact 14.
In this case, then, the output of the voltage sensor is advantageously a current, which can be easily converted into a low-voltage signal. However, the presence of a transistor within the conduction region 5 introduces active parasitic components, with consequent problems as regards possible undesired activations (latch-up) and tends to modify the behavior of the power device. Furthermore, the overall dimensions increase.